1. Field
This application relates to methods for making thermoelectric materials, specifically to methods for producing nanowires or quantum dots embedded in a solid.
2. Prior Art
When a temperature difference is applied across a thermoelectric material, electrical charge diffuses to one of its ends. If the thermoelectric material is n-type then a net negative charge diffuses to its cold end. If the thermoelectric material is p-type then a net negative charge diffuses to its hot end. The magnitude of this net charge buildup on the thermoelectric material's ends is expressed by its Seebeck coefficient, S.
If an n-type and p-type thermoelectric materials are placed near each other and their hot ends are electrical connected and their cold ends are electrical connected, electric current will flow through the thermoelectric materials. This electric current can be harnessed to provide electric power.
If no temperature difference exists across electrically connected n-type and p-type thermoelectric materials, a temperature difference can be created by driving an electric current through them. Generally, up to a point, the higher the electric current, the greater the temperature difference created. This temperature difference can be used to cool an object.
Whether thermoelectric materials are used to generate electrical power or for cooling, some of the energy transfer involved is lost (i.e. unusable). In either case, a temperature difference exists across the thermoelectric materials so energy flows (heat) through them. This heat is lost. Heat loss through thermoelectric materials can be minimized by using thermoelectric materials with a low thermal conductivity, k. Furthermore, electric current flowing through the thermoelectric materials results in resistive power loss. This resistive power loss can be minimized by using thermoelectric materials with a high electrical conductivity, σ.
So qualitatively, it is understandable that S and σ should be high and k should be low for efficient power generation and cooling. Through a rigorous mathematical derivation it can be shown that the efficiency of a thermoelectric material is directly related to a dimensionless figure of merit, ZT, which is defined as: ZT=TS2σ/k, where T is temperature.
Thermoelectric devices have been limited to niche markets due to their low energy efficiency and high cost. ZT for commercial thermoelectric materials is close to 1 at room temperature; however a ZT ≧2 is needed to compete with the energy efficiencies of most conventional cooling machines (i.e. automobile air-conditioners, small household refrigerators, etc.). Commercial thermoelectric devices are expensive because one of their primary constituents, tellurium, is a relatively rare and expensive element that is in high demand due to the rapid growth in cadmium telluride solar cell production. Given the continued global support for clean energy technologies (i.e. CdTe solar cells), there is an urgent economic need to replace tellurium based thermoelectric materials with a less expensive material that exhibits a ZT ≧1.
The most promising approach for increasing ZT of a material is to make it into a nanostructured form. In 1993, Hicks and Dresselhaus (“Thermoelectric Figure of Merit of a One-Dimensional Conductor”, Physical Review B Vol. 47, pp. 16631, 1993) pointed out that, due to the quantum-mechanical nature of the motion of electrons through solids, confining such electrons in a structure with a physical dimension below the spatial extent of the electron wave function should result in a much increased S and σ, thus strongly increasing ZT (later it was found that nanostructures also reduce k due to phonon scattering). For example a figure of merit ZT=6 (at 77° K) is predicted for 5 nm diameter bismuth nanowires oriented parallel to a temperature gradient. Humphrey et. al. (“Reversible Thermoelectric Nanomaterials”, Physical Review Letters, PRL 94, 096601, 2005) calculated ZT=10 (at 300° K) for nanowires oriented perpendicular to a temperature gradient.
After Hicks and Dresselhaus published their work in 1993, many attempts were made to fabricate thermoelectric nanostructures using a variety of methods. The results were disappointing primarily because of the costly and slow fabrication methods used and by the fact that bulk quantities of thermoelectric nanostructures must be embedded in a solid for structural support. This solid or host material is usually not a thermoelectric material so it tends to leak electric current and heat which significantly reduces ZT of the nanostructure-host composite. Generally the electrical current leakage is not a problem because thermoelectric nanostructures are made of a semiconductor so the host material can be made of an electrical insulator (i.e. thermoelectric material is Bi2Te3: σ˜1×105 (Ω·m)−1, host material is glass: σ˜1×10−12 (Ω·m)−1). However, the heat leakage is a problem because good thermoelectric materials tend to be thermal insulators (i.e. Bi2Te3: k˜1.2 W/(m·K), glass: k˜1 W/(m·K)). This heat leakage (or heat loss) becomes more significant by the fact that the thermal conductivity of nanostructures decreases dramatically as their size decreases: see for example FIG. 1 in Walkauskas et. al (“Lattice thermal conductivity of wires”, Journal of Applied Physics, Vol. 85, No. 5, 2579, 1999) and Zhou et. al. (“Thermoelectric properties of individual electrodeposited bismuth telluride nanowires”, Applied Physics Letters, 87, 133109, 2005). Increasing the number of thermoelectric nanostructures in a given volume of the host material helps to reduce this heat leak, however if the thermoelectric nanostructures are too close to one another, electron tunneling between adjacent thermoelectric nanostructures reduces ZT of the nanostructure-host composite as calculated by Broido and Reinecke (see FIG. 1 in “Thermoelectric figure of merit of quantum wire superlattices”, Applied. Physics Letters, 67 (1), 100, 1995). Clearly there is a need to use very low thermal conductivity materials to host thermoelectric nanostructures.
Making wires, with diameters less than 1 urn, embedded in glass, can be produced with the Taylor-wire process (see Section 2.2.1 and FIG. 1 in I. W. Donald “Production, properties and applications of microwire and related products,” Journal of Materials Science, Vol. 22, 2661-2679, 1987). Donald describes the Taylor-wire process as follows: “ . . . the metal to be drawn was contained in a glass tube ˜2 mm internal diameter, which was closed at one end. This end of the tube was heated in a gas flame until the metal melted and the glass softened, when the tube was drawn down, by hand, to produce metal-filled rods ˜0.5 to 1.0 mm diameter by about 300 mm long. Subsequently, these rods were re-drawn to the diameter required . . . ”. In 2005, Dutta (U.S. Pat. No. 7,559,215) used the Taylor-wire process in his attempt to make wire with the “diameter required” for high ZT thermoelectric materials.
The problem with the wires that Dutta produced is that the host material, in which Dutta's wires were embedded, was a glass. In fact, as stated in all Dutta's independent claims, glass is the material used to host his wires. Although glass is generally an electrical insulator, a much better thermal insulator is needed to host low thermal conductivity thermoelectric wires. For example, polymers have thermal conductivities that are typically ¼ to 1/20 that of glasses (see FIG. 17.5 on page 652 in “Properties of polymers: their correlation with chemical structure; their numerical estimation and prediction from additive group contributions” e.d. D.W. Van Krevelen, Amsterdam, New York: Elsevier, 1990, and FIG. 8.5 on page 149 in “From Polymers to Plastics”, A. K. van der Vegt, Delft, The Netherlands, Delft University Press, 2002). Dutta did not realize the large heat leak through the glass that hosted his wires, which, in turn, lead to his erroneous calculations of ZT that he states in his U.S. Pat. No. 7,559,215.
In research not related to thermoelectric materials, Bayindir et. al. (see inset of FIG. 1a in “Metal-insulator-semiconductor optoelectronic fibres,” Nature, Vol. 431, 826, 2004, and U.S. Pat. No. 7,295,734 filed in 2004) developed a method for making optoelectronic fibers that included microwires embedded in a polymer or glass, by using a conventional optical fiber drawing process. A couple of years later Bayindir (see FIG. 3d in “Kilometer-Long Ordered Nanophotonic Devices by Preform-to-Fiber Fabrication,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 12. no. 6, 1202, 2006, also see US Patent Application No. 2008/0087047) refined his method further by making photonic bandgap optical fibers containing 200 nm diameter nanowires embedded in a polymer. Bayindir's fibers were designed to function as photonic or optoelectronic devices which are very different from thermoelectric devices.
For making nanowires, Bayinder's method has two advantages over Dutta's use of the Taylor-wire process. First, Bayinder's method is compatible with polymer hosts which typically have much lower thermal conductivities than that of glasses. Although drawing a polymer tends to increase its thermal conductivity parallel to the draw direction, and decrease its thermal conductivity perpendicular to the draw direction, this thermal conductivity anisotropy can be significantly reduced by drawing the fiber at higher temperatures as demonstrated by Washo et. al. (see FIG. 2 in “Heat Conduction in Linear Amorphous High Polymers: Orientation Anisotropy,” Journal of Applied Physics, Vol. 40, no. 6, 2423, 1969″). Second, in Bayinder's method the nanowire host can be heterogeneous, such as assuming the form of nanotubes which further reduces the host's thermal conductivity due to phonon scattering at the nanotubes' inner and outer surfaces, i.e. the tube wall is like a thin polymer film that scatters phonons as measured by Jin et. al. (see Table I in “In-plane thermal conductivity of nanoscale polyaniline thin films”, Applied Physics Letters, 95, 033113, 2009).
So the method that Bayindir uses to make nanowires for optoelectronic and photonic devices in fibers could have a new use, namely, using nanowires embedded in a polymer or combination of polymers to serve as high ZT thermoelectric materials.
Furthermore, by pushing Bayindir's method of making nanowires beyond its intended operating parameters, the nanowires can be made to uniformly breakup into a well ordered, three dimensional array of quantum dots, which is the theoretically best (i.e. highest ZT) thermoelectric nanostructure. For ZT calculations on quantum dots see Mahan et. al. (“The best thermoelectric”, Proc. Natl. Acad. Sci., Vol. 93, 7436-7439, 1996) and Humphrey et. al. (“Reversible Thermoelectric Nanomaterials”, Physical Review Letters, PRL 94, 096601, 2005).
Furthermore, as previously noted, drawn polymers exhibit anisotropy in their thermal conductivity so if a fiber containing quantum dots is oriented such that the direction in which it was drawn is perpendicular to the applied temperature gradient, the fiber's ZT will be further increased because the fiber's thermal conductivity is lowest in this orientation.
Although several researches have successfully made nanoparticle chains in nanotubes and nanowires, the methods they employed are not suitable for producing bulk quantities of nanostructured thermoelectric materials. For example Qin et. al. (“General Assembly Method for Linear Metal Nanoparticle Chains Embedded in Nanotubes”, Nano Letters, Vol. 8, No. 10, 3221-3225, 2008, and “Rayleigh-Instability-Induced Metal Nanoparticle Chains Encapsulated in Nanotubes Produced by Atomic Layer Deposition”, Nano Letters, Vol. 8, No. 1, 114-118, 2008) used a slow atomic layer deposition process to make nanoparticles in nearly parallel, but short, and irregularly spaced nanotubes. Peng et. al. (“Bulk-quantity Si nanosphere chains prepared from semi-infinite length Si nanowires”, Journal of Applied Physics, Vol. 89, No. 1, 727-731, 2001) slowly grew short, randomly oriented nanowires containing nanospheres.
The problems with the methods Qin and Peng use to make nanoparticle chains is: 1) they use slow atomic layer deposition or growth methods, 2) the spacing between their nanoparticle chains is not well controlled which is important for achieving a high ZT thermoelectric material because if this spacing is too large then too much heat is leaked and if this spacing is too small then electron tunneling occurs, and 3) only short lengths and small quantities of nanoparticles can be produced.
So pushing Bayindir's method of making nanowires beyond its intended operating parameters, quantum dots embedded in a polymer or combination of polymers can be made to serve as high ZT thermoelectric materials.